When audio signals are to be transmitted and/or stored, a standard approach today is to code the audio signals into a digital representation according to different schemes. In order to save storage and/or transmission capacity, it is a general wish to reduce the size of the digital representation needed to allow reconstruction of the audio signals with sufficient quality. The trade-off between size of the coded signal and signal quality depends on the actual application.
There is a large variety of different coding principles. Transform based audio coders compress audio signals by quantizing the transform coefficients. Such coding thus operates in a transformed frequency domain. Transform based audio coders are efficient concerning moderate and high-bitrate coding of general audio but are not very efficient concerning low-bitrate coding of speech.
Code-Excited Linear Prediction (CELP) codecs, e.g. Algebraic Code-Excited Linear Prediction (ACELP) codecs, are very efficient at low bit-rate speech coding. The CELP speech synthesis model uses analysis-by-synthesis coding of the speech signal of interest. The ACELP codec can achieve high-quality at 8-12 kbit/s. However, signal features having high-frequency components are generally not modeled equally well.
One approach used for reducing the required bit-rate is to use BandWidth Extension (BWE). The main idea behind BWE is that part of an audio signal is not transmitted, but reconstructed (estimated) at the decoder from the received signal components. A combination of a CELP coding of a signal sampled by a low sampling rate and BWE is one solution that is discussed.
On the other hand BWE is more efficiently performed in a transformed domain, e.g. a Modified Discrete Cosine Transform (MDCT) domain. The reason for this is that the perceptually important signal features in the BWE region is more efficiently modeled in a frequency domain representation.
A problem with prior art codec systems is thus to find BWE encoding schemes that are efficient for all types of audio signals.